Embodiments of the subject matter disclosed herein generally relate to systems for mechanical power production from heat recovery.
Organic Rankine cycle (ORC) system have been used, as alternative to common water-steam Rankine cycle system, for waste heat recovery processes and are used for driving electric generators, thus transforming waste heat into useful electric energy.
FIG. 1 illustrates a combined gas turbine-ORC system for electric energy production. An organic Rankine cycle system is used for recovering waste heat from a gas turbine exhaust and converting it into electric energy by means of a double closed loop system. Reference number 101 indicates a gas turbine, e.g. used as a prime mover for an electric generator, schematically shown at 102. The exhaust combustion gases of the gas turbine 101 are cooled in a heat exchanger 103 and heat is extracted therefrom by means of a first closed loop 104. In the first closed loop 104 a diathermic oil is used as a heat transfer fluid to transfer the heat released by the gas turbine 101 to a fluid circulating in a second closed loop 105. The first closed loop 104 comprises a pump 106 and three serially arranged heat exchangers 107, 108 and 108a, where heat is transferred from the diathermic oil to the fluid circulating in the second closed loop 105.
The second closed loop 105 is a thermodynamic cycle based on the Rankine principle, wherein the working fluid is an organic fluid, in particular a heavy hydrocarbon, e.g. cyclopentane, or a refrigerant fluid with suitable characteristics.
The working fluid circulating in the second closed loop 105 is heated, vaporized and superheated in the three serially arranged heat exchangers 108a, 108 and 107, respectively, and expands in a turboexpander 109. The thermodynamic characteristics of the fluid allow a dry expansion to take place, i.e. the fluid at the discharge side of the turboexpander 109 is still in a gaseous state. A recuperator 110 is positioned downstream of the turboexpander 109. In the recuperator 110 the hot expanded fluid exchanges heat with the cold, high-pressure liquid obtained by condensing the expanded fluid in a condenser 112 and then pumping the condensate at the required upper pressure of the thermodynamic cycle by means of a pump 113. The liquid delivered by the pump 113 is pumped through the recuperator 110 and then the preheater, the vaporizer and the superheater exchangers 108a, 108, 107 closing the loop.
The turboexpander 109 is mechanically connected to an electric generator 115, which converts the mechanical power available on the output shaft of the turboexpander 109 into electric power.
The diathermic oil and the organic fluid circulating in the two closed loops allow low temperature heat sources to be exploited efficiently to produce electricity energy over a wide range of power output.